Hess's law: delta_H is path-independent. Three calculation methods: (1) Formation enthalpies: delta_H = sum(ndelta_Hf products) - sum(ndelta_Hf reactants). Elements in standard state have delta_Hf = 0. (2) Bond energies: delta_H = sum(BE bonds broken) - sum(BE bonds formed). Breaking bonds is endothermic (+), forming is exothermic (-). (3) Combustion enthalpies: delta_H = sum(ndelta_Hc reactants) - sum(ndelta_Hc products). Note the reversal compared to formation! Common error: mixing up the sign convention between formation (products - reactants) and bond energy (reactants - products for bonds).